A gravity gradiometer is disclosed in our International Patent Application No. PCT/AU2006/001269 and several concurrently filed applications. The content of International Patent Application No. PCT/AU2006/001269 is incorporated into this specification by this reference.
Gravimeters are widely used in geological exploration to measure the first derivatives of the earth's gravitational field. Whilst some advances have been made in developing gravimeters which can measure the first derivatives of the earth's gravitational field because of the difficulty in distinguishing spatial variations of the field from temporal fluctuations of accelerations of a moving vehicle, these measurements can usually be made to sufficient precision for useful exploration only with land-based stationary instruments.
Gravity gradiometers (as distinct from gravimeters) are used to measure the second derivative of the gravitational field and use a sensor which is required to measure the differences between gravitational forces down to one part in 1012 of normal gravity.
Typically such devices have been used to attempt to locate deposits such as ore deposits including iron ore and geological structures bearing hydrocarbons.
The above-mentioned gradiometer has a sensor in the form of a sensor mass which is pivotally mounted for movement in response to the gravity gradient.
Typically gravity gradiometers have a transducer which includes a sensing coil formed by winding a wire onto a substrate. The so-formed sensing coil is located in proximity to a moveable sensor mass so that when the mass moves relative to the coil, the magnetic flux changes to in turn change the current flowing through the coil which is used to enable a SQUID device to provide a measure of the gravity gradient.
Typically the sensing coils are required to have a large number of turns which can make the coils difficult and expensive to manufacture, particularly given the size constraints which are usually applicable to gravity gradiometers.